The applicant has proposed a low-pressure furnace for metal casting and has filed it as a patent application. The furnace can pressurize the molten metal in a pressure chamber with high precision. Also, it can greatly reduce the usage of inert gas, compared to a conventional furnace. The low-pressure furnace puts a pressure chamber next to a compartment for holding the molten metal wherein the chamber can communicate with the compartment. An isolation valve for molten metal is placed where the compartment and the pressure chamber are in communication with each other. A conduit for communicating is arranged downstream of the pressure chamber so that it can communicate with the sprue at the end of the conduit. A float is provided in the pressure chamber. The float is moved up and down by a electric cylinder that transforms the revolving movement to a linear movement via a screw mechanism. A pipe is provided on the pressure chamber. An inert gas is supplied to exhaust air in the pressure chamber through the pipe. (See Japanese Patent Laid-open Publication No. 2006-122910.)
However, in this conventional low-pressure furnace for metal casting, after the float is raised, when the casting that is a result of solidification of the molten metal is taken out of the mold, air comes through the mold into the conduit. Accordingly, the surface of the molten metal in the conduit is oxidized and an oxidized film forms on it. In addition, when the casting is taken out of the mold, the molten metal that has been maintained in the conduit up to the same height of a sprue rapidly falls by means of the atmospheric pressure. Therefore, the molten metal at the end of the conduit is agitated. As a result, it causes problems such as the oxidized film getting into a casting that is being cast. This causes a defective casting.
The present invention is aimed to resolve this problem. Namely, it is aimed to provide a low-pressure casting apparatus and a method for filling it with an inert gas, which can prevent the molten metal that is a residue in a conduit from being oxidized. A conduit in the apparatus is connected to a lower part of a pressure chamber in which the molten metal is pressurized. Also, the end of the conduit is made so that it can communicate with a sprue for a mold.